Method for producing glass substrate for magnetic disk and method for manufacturing magnetic disk

ABSTRACT

When mirror polishing is performed on a glass substrate by bringing a polishing pad into contact with the surface of the glass substrate while supplying a polishing liquid containing polishing grains to the substrate surface, the pH of the polishing liquid is maintained within a certain range or the agglomeration degree or dispersion degree of the polishing liquid is controlled. Consequently, an adequate mirror polishing rate can be maintained and there can be obtained a glass substrate having a good end shape.

This is a divisional of application Ser. No. 12/088,851 filed Mar. 31,2008, which is the National Stage Entry of PCT/JP2007/055420 filed onMar. 16, 2007, as well as the content of U.S. Application 60/785,283filed Mar. 24, 2006; Japanese Patent Application 2006-182550 filed Jun.30, 2006, and Japanese Patent Application 2006-182441 filed Jun. 30,2006; from which priority has been claimed in the prior application, isconsidered part of the disclosure of the accompanying Divisionalapplication and is hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to a method for producing a glass substrate formagnetic disks and also to a method for manufacturing a magnetic disk.

BACKGROUND ART

Magnetic disks are those magnetic recording media mounted in hard diskdrives. The magnetic disk is fabricated by successively laminating anunderlying layer, a magnetic layer, a protective layer and a lubricantlayer on a disk-shaped substrate.

One of important parts of the magnetic disk is a substrate. A magneticfilm and the like are formed on a substrate as reflecting a surfaceprofile of the substrate, so that the surface profile of the magneticdisk is determined depending of the surface profile of the substrate.

With hard disk drives, a magnetic head serving as an informationrecording and reproducing means is moved over a magnetic disk serving asan information accommodation means at high speed while keeping a narrowfloating gap, thereby permitting the information to be written or readthereon. The contact between the magnetic head and the magnetic disk maybring about a serious trouble. When the floating height of a magnetichead is decreased, the recording density of information recorded on amagnetic disk can be improved. In order to lessen the floating height ofa magnetic head, the magnetic disk has to be smooth on the surfacethereof.

Accordingly, for improving the performance of a hard disk drive, it isnecessary that the surface of a magnetic disk be smooth. In order toenable the surface of a magnetic disk to be smoothed, the surface of asubstrate should be smooth.

From this standpoint, a glass substrate is very useful as a substratefor magnetic disk which is mounted in hard disk drives. This is for thereason that the glass substrate can be made smooth.

With respect to glass substrates for magnetic disk, there is known, forexample, the following document. For instance, JP-A-H07-240025, which isa Japanese Unexamined Patent Application Publication, is known.

In this document, a super polishing process wherein a disk substrate ispolished on the surface thereof is disclosed. More particularly, it isdisclosed that a sulfuric acid solution of colloid silica slurry isadjusted to an acidic pH, for example, of about 0.6 to 0.9, followed bypolishing a glass substrate. In this document, it is also disclosed thatit is important to note that a final pH and component concentrations areimportant in controlling a rate of removing a substrate member from adisk substrate. This document discloses a magnetic disk substratecomprising a substrate member having a surface roughness of smaller than4 Å. It will be noted that documents like this include U.S. Pat. Nos.6,236,542 and 6,801,396.

As other document, there is known JP-A-H10-241144 that is a JapaneseUnexamined Patent Application Publication. In this document, there isdisclosed a technique wherein using, for example, a colloidal silicapolishing liquid, a glass substrate for information recording medium ispolished. It is to be noted that documents like this includes U.S. Pat.Nos. 6,277,465 and 6,877,343.

Likewise, there is known JP-A-2004-063062 that is a Japanese UnexaminedPatent Application Publication. In this document, in the polishing of aglass substrate for information recording media, there is used, as apolish, a suspension of particles made of silicon dioxide (SiO₂) as amain component and having an average size of 100 nm or below. Thepolishing treatment is carried out according to two steps including thesteps of polishing a glass substrate with an acidic polish having a pHof 4 or below and polishing the glass substrate with an alkaline polishhaving a pH of not less than 8.5. It will be noted that a document likethis includes U.S. Published Patent Application No. 2003/0228461.

-   Patent Document 1: JP-A-H07-240025-   Patent Document 2: U.S. Pat. No. 6,236,542-   Patent Document 3: U.S. Pat. No. 6,801,396-   Patent Document 4: JP-A-H10-241144-   Patent Document 5: U.S. Pat. No. 6,277,465-   Patent Document 6: U.S. Pat. No. 6,877,343-   Patent Document 7: JP-A-2004-063062-   Patent Document 8: U.S. Published Patent Application No.    2003/0228461

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

In recent magnetic disks, need is involved in storing information at arecording density, for example, of as high as 100 gigabits or over perunit square inch. Although hard disk drives have been long utilized asan external memory of personal computers, their range of utility as astorage for recording video signals has been abruptly enlarged in recentyears. Therefore, information amount to be stored in hard disk driveshas increased rapidly.

For a first means for responding this need, there is mentioned a methodwherein an information storage capacity per unit area on the surface ofa magnetic disk is increased to effectively use a limited disk area.

In order to realize such a high recording density, is becomes necessaryto set a floating height of a magnetic head, for example, at 8 nm orbelow. This is because a more reduced floating height of the magnetichead leads to a more improved S/N ratio of recording signals. To thisend, it is needed that a glide height of a magnetic disk be at 4 nm orbelow. The “glide height of a magnetic disk at 4 nm or below” means thatif a magnetic head is allowed to float and fly over a magnetic disk at afloating height of 4 nm, there occurs no contact with the magnetic disk,thereby not causing a crashing failure.

Another second means includes a method of enlarging an informationrecording and reproducing area of a magnetic disk. When the recordingand reproducing area is extended to the vicinity of an outer edge, aninformation amount stored in one magnetic disk can be efficientlyincreased. This brings about a necessity of not causing a crash failurewithout contact of the magnetic head with the magnetic disk even at thevicinity of an outer edge of the main surface of the magnetic disk.

As a further third means, there is a method using a vertical magneticrecording system as a recording system of a magnetic disk. The verticalrecording system is one wherein recorded magnetization is oriented alonga direction normal to a plane of a disk. The orientation of recordedmagnetization in a direction normal to the disk plane permits themagnetization in the vicinity of a bit boundary to be stabilized. Forthis, the vertical magnetization recording system is more advantageous,as a recording system corresponding to high recording density, overin-plane magnetic recording systems. For a magnetic disk correspondingto a vertical magnetic recording system, a so-called perpendiculardouble-layered medium wherein a soft magnetic layer is interposedbetween a magnetic recording layer and a substrate is beneficial.

Recently, hard disk drives have been in frequent use for portablepurposes. For instance, they are, in many cases, mounted in mobiledevices to be frequently moved from one to another such as portableinformation terminals, car navigation systems, cellular phones and thelike. The hard disk drive of such a use as mentioned above is limited insize, for which a magnetic disk used is of a small size. The small-sizedmagnetic disk includes, for example, a 1.8-inch magnetic disk, a 1-inchmagnetic disk, a 0.85-inch magnetic disk or the like.

Such small-sized hard disk drives have a small main surface area of amagnetic disk, for which there is especially a great need for means thatensures a high information capacity. In addition, there is also a greatneed for high impact resistance because they are designed as a portablehard disk drive and thus, have a great possibility of being exposed tovibrations and impact shock.

Further, there is a great need for mass production and low pricerelative to magnetic disks and substrates therefor. As set outhereinabove, this is because hard disk drives are advantageous in thatthey are high in capacity and excellent in portability and can be madesmall in size, and thus, the market has been sharply grown since 2005.

Glass substrates are especially preferred for use as a substrate formagnetic disks, which satisfies the needs for such hard disk drives. Thereason for this is that a glass substrate is able to provide excellentsmoothness when subjected to mirror polishing, and can effectively copewith a problem on a small floating height of a magnetic head. Inaddition, since rigidity is high, an impact resistance becomesexcellent.

Nevertheless, if a glass substrate for magnetic disks, which isespecially suited for the need of recent hard disk drives, ismass-produced, many troubles are brought about. In particular, if theprocess of making a glass substrate for magnetic disk corresponding to alow glide height is adapted to mass production, it becomes necessary toinnovate the process of mirror polishing a main surface of the glasssubstrate.

For example, if it is intended to set the glide height at 4 nm or below,it takes a very long processing time for the mirror polishing of theglass substrate surface. This makes it difficult to ensure asatisfactory amount of production. The provision of a glass substratefor magnetic disk that is high in quality and inexpensive becomesdifficult. When a glass substrate for magnetic disk having a smoothsurface corresponding to a low glide height is made, the floating of amagnetic head is prone to become instabilized in the vicinity of anouter edge of the disk along with concern that expansion of aninformation recording and reproducing area is impeded.

Further, it has been found that in the polishing methods disclosed inthe above-indicated documents, the polishing rate lowers during thecourse of the polishing, with some cases where the productivity of theglass substrate for magnetic disk is worsened. The prior art polishingmethods have had the drawback that the resulting glass substrate isworsened in end shape. Accordingly, if the floating height of a magnetichead is lowered, the magnetic head is in contact with an end portion ofa magnetic disk, sometimes resulting in crashing.

The invention has been accomplished so as to solve such problems as setforth above and a first object of the invention is to provide a magneticdisk capable of achieving an information recording density of not lessthan 100 gigabits per unit square inch and a glass substrate for themagnetic disk.

A second object of the invention is to provide a magnetic disk thatpermits a floating height of a magnetic head to be at 8 nm or below anda glass substrate for such magnetic disk.

A third object of the invention is to provide a magnetic disk capableachieving a glide height of 4 nm or below and a glass substrate for themagnetic disk.

A fourth object of the invention is to provide a magnetic disk wherein amagnetic head is able to record and reproduce information even in thevicinity of an outer edge of the magnetic disk and a glass substrate forthe magnetic disk.

A fifth object of the invention is to provide a magnetic diskcorresponding to a vertical magnetic recording system and a glasssubstrate for the magnetic disk.

A sixth object of the invention is to provide a small-sized magneticdisk such as 1.8-inch, 1.0-inch and the like disks and a glass substratesuited for the magnetic disk.

A seventh object of the invention is to provide a magnetic disk suitedfor mass production and a glass substrate for the magnetic disk.

Another object of the invention is to provide a method for making aglass substrate for magnetic disk, which ensures high productivitywithout lowering a polishing rate during the polishing and a method formanufacturing a magnetic disk.

A further object of the invention is to provide a method for making aglass substrate for magnetic disk having a good end shape.

Measure for Solving the Problems

The invention is directed to those inventions including at least thefollowing inventive arrangements.

(Inventive Arrangement 1)

A method for making a glass substrate for magnetic disk including amirror polishing treatment of a glass substrate, wherein the mirrorpolishing treatment includes contacting a polishing pad with a surfaceof a glass substrate, supplying a polishing liquid containing polishinggrains to the surface of the glass substrate, subjecting the glasssubstrate and the polishing pad to relative movement to mirror polishthe surface of the glass substrate and that when a plurality of glasssubstrates are subjected to the mirror polishing treatment, thepolishing liquid is kept at a certain pH.

(Inventive Arrangement 2)

A method for making a glass substrate for magnetic disk as recited inArrangement 1, wherein the polishing liquid is composed of an acidicpolishing liquid containing an inorganic acid and a buffer agent.

(Inventive Arrangement 3)

A method for making a glass substrate for magnetic disk as recited inArrangement 1 or 2, wherein the polishing liquid is composed of anacidic polishing liquid containing an organic acid.

(Inventive Arrangement 4)

A method for making a glass substrate for magnetic disk as recited inany one of Arrangements 1 to 3, wherein the glass substrate is made ofglass having a glass skeleton of a network structure and a modifying ionmodifying the network structure.

(Inventive Arrangement 5)

A method for making a glass substrate for magnetic disk as recited inany one of Arrangements 1 to 4, wherein a pH of the polishing liquid iskept at 3 or below during the course of the mirror polishing treatment.

(Inventive Arrangement 6)

A method for making a glass substrate for magnetic disk as recited inany one of Arrangements 1 to 5, wherein the polishing liquid containssulfuric acid.

(Inventive Arrangement 7)

A method for making a glass substrate for magnetic disk as recited inany one of Arrangements 1 to 6, wherein the polishing liquid containstartaric acid or maleic acid.

(Inventive Arrangement 8)

A method for making a glass substrate for magnetic disk as recited inany one of Arrangements 1 to 7, wherein the polishing grains includecolloidal silica particles.

(Inventive Arrangement 9)

A method for making a glass substrate for magnetic disk as recited inany one of Arrangements 1 to 8, wherein the mirror polishing treatmentincludes moving a plurality of glass substrates, sandwiched between anupper platen and a lower platen through the polishing pads, relative tothe upper platen and lower platen to mirror polish opposite surfaces ofthe plurality of glass substrates at the same time.

(Inventive Arrangement 10)

A method for making a glass substrate for magnetic disk as recited inany one of Arrangements 1 to 9, wherein the platens are, respectively,made of a material having a corrosion resistance against an acid.

(Inventive Arrangement 11)

A method for making a glass substrate for magnetic disk as recited inany one of Arrangements 1 to 10, the method further including apre-polishing treatment for pre-polishing the glass substrate prior tothe mirror polishing treatment, wherein the pre-polishing treatmentincludes contacting a polishing pad on a surface of the glass substrate,supplying a polishing liquid containing polishing grains to the surfaceof the glass substrate, and relatively moving the glass substrate andthe polishing pad thereby mirror polishing the surface of the glasssubstrate wherein the polishing grains contained in the polishing liquidused in the pre-polishing treatment include cerium oxide polishinggrains from which grains having a size of not smaller than 4 nm areremoved.

(Inventive Arrangement 12)

A method for making a glass substrate for magnetic disk as recited inany one of Arrangements 1 to 11, the method further including apre-polishing treatment for pre-polishing the glass substrate prior tothe mirror polishing treatment, wherein the pre-polishing treatmentincludes contacting a polishing pad on a surface of the glass substrate,supplying a polishing liquid containing polishing grains to the surfaceof the glass substrate, and relatively moving the glass substrate andthe polishing pad thereby mirror polishing the surface of the glasssubstrate, wherein the polishing pad used in the pre-polishing treatmentis one that contains zirconium oxide and cerium oxide.

(Inventive Arrangement 13)

A method for manufacturing a magnetic disk, including forming magneticlayers formed on the glass substrate made according to a method formaking a glass substrate as recited in any one of Arrangements 1 to 12.

(Inventive Arrangement 14)

A method for manufacturing a magnetic disk as recited in Arrangement 13,wherein at least one layer of the magnetic layers is made up of a softmagnetic layer, thereby providing a vertical magnetic recording disk.

(Other Arrangement)

In other embodiment of the invention, there is provided a method formaking a glass substrate for magnetic disk which includes a mirrorpolishing step including contacting a polishing pad on a surface of amulti-component glass substrate, supplying a polishing liquid containingpolishing grains to a surface of the glass substrate and relativelymoving the glass substrate and the polishing pad to polish the surfaceof the glass substrate, wherein a pH value of the polishing liquid iskept within a certain range. In this case, the pH value of the polishingliquid is preferably kept at from 1 to 3.

In this embodiment, the polishing liquid should preferably contain aninorganic acid so as to make an acidic pH value of the polishing liquidand a buffer agent for keeping the pH value of the polishing liquidconstant. The inorganic acid is preferably sulfuric acid and the bufferagent is preferably an organic acid. The organic acid is more preferablytartaric acid or maleic acid.

In a further embodiment of the invention, there is provided a method formaking a glass substrate for magnetic disk including a mirror polishingstep which includes contacting a polishing pad with a surface of amulti-component glass substrate, supplying a polishing liquid containingpolishing grains to a surface of the glass substrate, and relativelymoving the glass substrate and the polishing pad to polish the surfaceof the glass substrate, wherein the mirror polishing procedure ispreferably carried out in such a way that the polishing liquid iscontrolled to have such a degree of coagulation or dispersion in thepolishing liquid to provide a Duboff value, indicating an end shape ofthe glass substrate obtained in the mirror polishing step, of ±10 nm.

In the practice of the invention, it is preferred that a zeta potentialof the polishing grains contained in the polishing liquid is not largerthan −10 mV or not less than +10 mV.

In the invention, the polishing liquid is preferably acidic. In the casewhere the pH value of the polishing liquid is at 2.0, the zeta potentialof the polishing grains is preferably not larger than −10 mV or not lessthat +10 mV. In case where the pH value of the polishing liquid is at3.0, the zeta potential of the polishing grains is preferably not largerthan −30 mV or not less than +30 mV. The polishing grains contained inthe polishing liquid are preferably made of colloidal silica particles.

Further, in the practice of the invention, the polishing grainscontained in the polishing liquid are preferably made of colloidalsilica particles.

In the invention, the glass substrate preferably contains a glassskeleton of a network structure and a modifying ion modifying thenetwork structure.

In the invention, the glass substrate preferably includes, as maincomponents, from 58 wt % to 75 wt % of SiO₂, from 5 wt % to 23 wt % ofAl₂O₃, from 3 wt % to 10 wt % of Li₂O, and from 4 wt % to 13 wt % ofNa₂O.

In the invention, the mirror polishing step preferably includessandwiching the glass substrate between an upper platen and a lowerplaten through the polishing pad, supplying a polishing liquidcontaining polishing grains to a surface of the glass substrate, andrelatively moving the glass substrate and the upper and lower platens tomirror polish the surface of the glass substrate.

In the invention, the upper platen and the lower platen are preferablymade of a material having a corrosion resistance against acids,respectively.

In the invention, it is preferred to further include, prior to themirror polishing step, a pre-polishing step for pre-polishing the glasssubstrate on the surface thereof, the pre-polishing step includingcontacting a polishing pad with the glass substrate on the surfacethereof, supplying a polishing liquid containing polishing grains to thesurface of the glass substrate, and relatively moving the glasssubstrate and the polishing pad to polish the surface of the glasssubstrate wherein the polishing grains in the pre-polishing step aremade of cerium oxide particles having a grain size of smaller than 4 μm.In addition, the polishing pad used in the pre-polishing step preferablycontains zirconium oxide particles or cerium oxide particles.

The method for manufacturing a magnetic disk according to the inventionincludes forming a magnetic film on the glass substrate made by use ofthe method for making a glass substrate for magnetic disk. Moreover, themethod for manufacturing a magnetic disk according to the inventionincludes forming at least one soft magnetic layer on the glass substrateto obtain a vertical magnetic recording disk.

Effects of the Invention

According to the invention, there can be provided manufacturing methodsincluding a method for making a glass substrate for magnetic disk and amethod for manufacturing a magnetic disk, which are able to secure highproductivity without lowering a polishing rate in the midway of thepolishing treatment.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view of a polishing device for carrying out amethod for making a glass substrate for magnetic disk according to oneembodiment of the invention.

FIG. 2 is a chart indicating, as a table, the relationship between thepH value of a polishing liquid and the polishing rate in Examples 1 to 6according to another embodiment of the invention.

FIG. 3 is a chart indicating, as a table, the relationship between thezeta potential of polishing grains and the end shape of a glasssubstrate 1 after having been subjected to a mirror polishing step inReferences 1 to 4 of the invention.

FIG. 4 is a chart indicating, as a table, the relationship between thezeta potential of polishing grains and the end shape of a glasssubstrate after carrying out a mirror polishing step related to Examples1 to 3 according to a further embodiment of the invention andComparative Example 1.

FIG. 5 is a chart indicating, as a table, the relationship between thepH value of a polishing liquid and the polishing rate related toReferences 1 to 6 of the invention.

DESCRIPTION OF REFERENCE NUMERALS

-   1 glass substrate-   2 polishing pad-   3 a upper platen-   3 b lower platen-   10 polishing device

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

Amorphous glass is suited as a glass substrate for magnetic disk. Thisis because amorphous glass is made very smooth on the surface thereof bymirror polishing unlike, for example, crystallized glass and glassceramics.

Multi-component glasses such as aluminosilicate glass and the like arepreferably used as a glass material of the glass substrate for magneticdisk. Of glasses, aluminosilicate glass has a feature in that it isbetter in heat and chemical resistances than, for example, borosilicateglass. Accordingly, upon exposure to a chemical solution such as in acleaning step, there is a reduced possibility of a mirror-polishedsurface being roughened excessively and thus, such a glass is suitablefor use as a glass substrate for magnetic disk, in which smoothness isespecially required. The aluminosilicate glass means a glass containingoxides of silicon and aluminium as main components.

Although a glass substrate is suited for magnetic disks, a difficultyhas been involved in increasing a processing rate of mirror polishingtreatment to ensure mass production. Accordingly, limitation is placedon an production amount, for which production costs are liable to risewith a difficulty in low-cost supply on the market.

The present inventors have made intensive studies on these problems anddiscovered that a hydrogen ion concentration (pH value) of a polishingliquid varies in the course of a polishing treatment of a glasssubstrate, which is one of factors of lowering a polishing rate. It hasalso been discovered as one of factors that the variation of the pHvalue occurs due to the eluation, in an acidic solution, of ionscontained in the glass substrate.

The variation of the pH value of the polishing liquid is apt to occurespecially, when a multi-component glass substrate is polished. Forinstance, when an aluminosilicate glass substrate is polished with anacidic polishing liquid containing colloidal silica polishing grains, analuminium ion is sometimes dissolved out from the multi-component glasssubstrate in the acidic polishing liquid, thereby causing the pH valueof the polishing liquid to be varied. Besides, if glass to be polishedcontains sodium, potassium and the like, there is the possibility thatsuch sodium, potassium and the like ions are dissolved out in thepolishing liquid, thereby causing the pH value of the polishing liquidto be varied.

It will be noted that as a method of mirror polishing a glass substratefor magnetic disk, there may be some cases where a polishing liquidcontaining colloidal silica polishing grains is adjusted to acidity oralkalinity, followed by supplement and polishing. For example,JP-A-H07-240025 mentioned in the Technical Background is one instance ofpolishing after adjusting a slurry containing colloidal silica to agiven acidity.

According to the studies made by the present inventors, it has beenfound that especially, with a multi-component glass, an ion contained inthe glass is likely to dissolve out in a polishing liquid in the courseof a mirror polishing treatment. With aluminosilicate glass, analuminium ion is liable to dissolve out. If sodium, potassium and thelike are contained in the glass, there is concern that sodium, potassiumand the like ions may dissolve out in the polishing liquid.

If a polishing liquid is adjusted to acidity, the liquidity of thepolishing liquid is more apt to be disturbed as these ions are dissolvedout. According to the studies of the present inventors, it has beenfound that when an aluminosilicate glass substrate is subjected to amirror polishing treatment with an acidic polishing liquid containingcolloidal silica polishing grains, the pH of the polishing liquid isliable to vary from a given pH during the course of mass production. Ithas also been found that the disturbance in liquidity of the polishingliquid results in the variation in processing rate of the mirrorpolishing treatment.

If the pH of the polishing liquid used in the polishing treatment isadjusted to a predetermined level, an initially adjusted pH is graduallychanged during the course of the polishing treatment of a plurality ofglass substrates. The present inventors found that when glass substratesare mass-produced, this change becomes non-negligible.

Among glass substrates, a glass substrate for magnetic disk should havea very smooth surface because a magnetic head passes at a high speedwhile keeping a small floating gap or amount. Accordingly, when a glasssubstrate for magnetic disk is subjected to mirror polishing treatment,it is necessary to keep a constant liquidity of the polishing liquid.

In order to allow a glass substrate for magnetic disk to bemass-produced, it is necessary that the polishing rate of the mirrorpolishing treatment be kept constant. To this end, it is very effectiveto maintain the liquidity of a polishing liquid constant in the mirrorpolishing treatment of a glass substrate for magnetic disk.

As a specific method of keeping the pH of a polishing liquid at aconstant level, it is effective to add, to the polishing liquid, aningredient for keeping the pH constant. A chemical solution keeping apolishing liquid at a given pH may be contained.

(Polishing Liquid)

It is preferred to keep the pH of a polishing liquid acidic. This isbecause to keep the polishing liquid acidic during the mirror polishingtreatment contributes to chemical change of a glass surface therebyimproving a polishing rate. Especially, where a multi-component glass isused as a material for a glass substrate 1 and the glass substrate 1 isimmersed in an acidic polishing liquid, metallic ions are likely torelease from the network structure of SiO thereby enabling a polishingrate to be improved.

For instance, a multi-component glass is preferred in the sense thatmetallic ions such as of aluminium, sodium, potassium and the like,which are modifying ions, are contained in the network structure of theSiO glass skeleton. When the polishing liquid is kept acidic, thesemetallic ions are likely to release from the network structure of SiO,thereby showing the action of improving a mirror polishing rate. On theother hand, the metallic ions are dispersed in the polishing liquid, sothat the pH value of the polishing liquid increases with time and itbecomes difficult to keep a desired mirror polishing rate. In thepractice of the invention, however, the mirror polishing rate can bekept as desired.

As an ingredient for keeping the pH of the polishing liquid constant, itis preferred to contain a buffer agent to the polishing liquid. Organicacids are preferred because of their buffering action. For an ingredientused to make the pH of the polishing liquid acidic, inorganic acids arepreferably used.

The pH of the polishing liquid is kept at 3 or below, preferably 2.5 orbelow, more preferably 2 or below. To keep the polishing liquid at suchan acidic level enables a mirror polishing rate of a glass substrate,particularly, a multi-component glass substrate or an aluminosilicateglass substrate, to be kept at a level suited for mass production.

The pH of the polishing liquid should not be excessively acidic. This isbecause there is high concern that a polishing device is corroded. Ifthe polishing device is corroded, there is concern that fine foreignmatters (e.g. rust) may be attached to the mirror polished glasssubstrate. The attachment of such foreign matters to the surface of theglass substrate adversely influences a magnetoresisatnce effect elementutilized as a reproducing element of a magnetic head, thereby bringingabout a thermal asperity error in information reproducing signals.

Colloidal silica polishing grains are suited as polishing grains formirror polishing of a glass substrate. In this connection, if the pH ismade excessively acidic, the chemical state of the colloidal silicabecomes instabilized and gelation is prone to occur. If colloidal silicais gelled, its function as polishing grains is lost.

Accordingly, it is preferred that the pH value of the polishing liquidis not made acidic in excess. More particularly, the pH value ispreferably 1.0 or over.

From the standpoints set out above, the pH of the polishing liquidranges from 1.0 to 3.0, preferably from 1.0 to 2.5 and more preferablyfrom 1.0 to 2.0.

(Method of Adjusting an Acidic Polishing Liquid) It is preferred thatfor adjusting a polishing liquid to acidity, an inorganic acid iscontained in the polishing liquid. Use of an inorganic acid having thecapability of full dissociation makes it easy to provide an acidiccondition where the pH ranges, for example, from 1.0 to 3.0. Thus, thisis beneficial for mirror polishing of glass substrates.

The inorganic acids include sulfuric acid, hydrochloric acid, nitricacid, boric acid, phosphoric acid, phosphonic acid, phosphinic acid andthe like. With an inorganic acid having intense oxidative power, such anacid is liable to cause corrosion of a polishing device, along withconcern that a thermal asperity failure may be caused.

In view of the above, the inorganic acid contained the polishing liquidpreferably includes sulfuric acid, phosphoric acid or phosphonic acid.Most preferably, sulfuric acid having relatively small oxidative poweris mentioned. Sulfuric acid whose oxidative power is small has thesmallest possibility of corroding a polishing device. Hence, thepossibility of causing the thermal asperity failure is smallest.Moreover, sulfuric acid is unlikely to evaporate or scatter in air, withthe attendant advantage that the concentration in the polishing liquidcan be readily kept constant when the mirror polishing treatment iscarried out. It will be noted that where liquidity is adjusted by use ofsulfuric acid, it is preferred that a sulfuric acid concentration in thepolishing liquid ranges, for example, from 0.05 wt % to 1.00 wt %.

(Liquidity Adjusting Method)

According to such knowledge of the present inventors as statedhereinabove, the ions contained in a glass substrate are dissolved outin the polishing liquid during the polishing step to vary the pH of thepolishing liquid. The variation of the pH value is one of factors oflowering a polishing rate. In order to prevent the variation of the pHof the polishing liquid in the course of the polishing step, it ispreferred to contain a buffer agent in the polishing liquid.

An organic acid is preferably used as a buffer material contained in thepolishing liquid. The buffer action imparted to the polishing liquidpermits the pH of the polishing liquid to be kept at a desired givenlevel. Where the pH of the polishing liquid is kept within a range offrom 1 to 3, especially from 1 to 2, it is preferred to choose tartaricacid, maleic acid or maloic acid. Of these, tartaric acid or maleicacid, particularly tartaric acid, is more preferred. It will be notedthat where tartaric acid is used as a buffer agent, the concentration oftartaric acid in the polishing liquid is preferably 0.05 wt % to 1.50 wt%.

The polishing liquid which is most preferred in the invention is onewhich includes colloidal silica polishing grains, sulfuric acid as aninorganic acid, and tartaric acid as an organic acid.

(Polishing Grains)

Where a polishing treatment (mirror polishing step) is carried out usinga polishing liquid, polishing grains are contained in the polishingliquid. For the polishing grains used in the mirror polishing step, itis preferred to use colloidal silica particles. In the practice of theinvention, the colloidal silica polishing grains have a grain size ofnot larger than 80 nm, preferably not larger than 50 nm. The use of suchfine polishing particles contributes to creating a smooth mirror surfaceadapted as a glass substrate of magnetic disk. The lower limit of thegrain size of the colloidal silica polishing grains can be determinedwhile taking a mirror polishing rate into account. For instance, thesize may be set at from 20 nm to 50 nm. The content of the colloidalsilica particles in the polishing liquid is preferably from 5 wt % to 40wt %.

(Zeta Potential of Polishing Grains)

The polishing grains dispersed in a polishing liquid has a zetapotential. If the zeta potential is closer to 0 mV than ±10 mV,polishing grains are liable to coagulate, thus resulting in poordispersability of the polishing grains in the polishing liquid. If thedispersability in the polishing liquid becomes poor, the fluidity of thepolishing grains in the vicinity of end portions of the glass substrate1 lowers in the polishing step, with the possibility that the end shapeof the glass substrate 1 after the polishing step having been carriedout is worsened.

The zeta potential may be either plus or minus depending on thecomposition of target particles. The zeta potential of colloidal silica(colloid-shaped silica) usable as polishing grains becomes minus at apH, for example, of 3 or over, close to zero at a pH ranging 2.0 to 3.0and plus under lower conditions (at a pH of 1.0 or below).

In the practice of the invention, the zeta potential of colloidal silicais preferably at −10 mV or below. In particular, colloidal silica shouldpreferably be so selected as to have a zeta potential of not larger than−10 mV in case where the pH of the polishing liquid is at 2 and a zetapotential of not larger than −30 mV in case where the pH of thepolishing liquid is at 3.

With respect to the liquidity of the polishing liquid according to theinvention, it is preferred that the electric conductivity of thepolishing liquid is adjusted to a range of 2 mS/cm to 10 mS/cm.

(Mirror Polishing Treatment)

In the practice of the invention, the mirror polishing treatment ispreferably carried out by a polishing method wherein a plurality ofglass substrates are collectively subjected to mirror polishingtreatment on both surfaces thereof according to a both-sided polishingmethod making use of a planetary gear mechanism. Because a number ofglass substrates can be finished as a uniform mirror surface on bothsurfaces thereof, the polishing rate of the mirror polishing treatmentdoes not vary, permitting mass production to be stably held. Inaddition, in the practice of the invention, it is possible to keep aconstant liquidity of the polishing liquid, thus maintaining the stablemass production.

In the invention, mirror polishing can be performed by use of apolishing liquid circulation and re-utilization-type polishingapparatus. More particularly, a polishing liquid that has been oncesupplied to a glass substrate surface is collected and may be againsupplied to a glass substrate surface after passage through a cleaningmeans such as of filtering or the like. In the invention, the pH of thepolishing liquid can be kept at a desired given level, so that such apolishing liquid can be re-used. There may occur, not in accordance withthe invention, a problem that the pH of a polishing liquid becomes proneto vary during the circulation and re-utilization of the polishingliquid.

In the mirror polishing treatment of a glass substrate for magneticdisk, the polishing liquid can be circulated and re-utilized, so that itbecomes possible to abate emissions of industrial waste. Thus, amass-production procedure of a glass substrate for magnetic disk can beestablished while consideration is given to the global environment.

In the practice of the invention, at least a polishing platen of themirror polishing apparatus should preferably be made of a material thathas a corrosion resistance against acids. Such materials preferablyinclude stainless steels. Stainless steels having an excellent corrosionresistance preferably include martensite stainless steels or austenitestainless steels.

(Glass Substrate)

The glass substrate related to the invention is constituted of glass(multi-component glass). The multi-component glass includes, forexample, a network structure of SiO that is a glass skeleton, andmetallic ions, such as of aluminium, sodium, potassium and the like,serving as a modifying ion therefor. Where such a multi-component glassis immersed in an acidic polishing liquid, the metallic ions are likelyto release from the network structure of SiO, thereby improving thepolishing rate. More particularly, the glass substrate is chemicallychanged on the surface thereof, enabling the polishing rate to beimproved.

In the invention, a preferred glass substrate is made of amorphous glassor aluminosilicate glass containing oxides of silicon and aluminium asmain components. This is because the amorphous glass ensures a verysmooth surface by polishing unlike, for example, crystallized glass orglass ceramics. Aluminosilicate glasses are superior in resistances toheat and chemicals to borosilicate glass and when it is exposed to achemical solution such as in a cleaning treatment, there is a reducedpossibility that the surface of the glass substrate 1 after polishing isroughened in excess.

Among aluminosilicate glasses, a glass substrate containing an alkalimetal element is preferred. For example, when those glasses containingSiO₂ and Al₂O₃ and further Na₂O are used, the effect of the inventioncan be satisfactorily shown. This is because if an aluminium ion orsodium ion is dissolved out in the polishing liquid, the pH of thepolishing liquid can be kept as desired. A glass containing Li₂O canalso be used as preferred. For instance, glass for chemical reinforcingtreatment is a type of glass containing an alkali metal element and canbe preferably used in the invention.

For such a glass (multi-component glass) as set out above, there ispreferably mentioned a glass including, as main components, 58 to 75 wt% of SiO₂, 5 to 23 wt % of Al₂O₃, 3 to 10 wt % of Li₂O and 4 to 13 wt %of Na₂O.

A more preferred glass is one (aluminosilicate glass), which includes,as main components, 62 to 75 wt % of SiO₂, 5 to 15 wt % of Al₂O₃, 4 to10 wt % of Li₂O, 4 to 12 wt % of Na₂O and 5.5 to 15 wt % of ZrO₂provided that a ratio by weight Na₂O/ZrO₂ is at 0.5 to 2.0 and a ratioby weight of Al₂O₃/ZrO₂ is at 0.4 to 2.5.

Another preferred glass includes 61 to 70 wt % of SiO₂, 9 to 18 wt % ofAl₂O₃, 2 to 3.9 wt % of Li₂O, 6 to 13 wt % of Na₂O, 0 to 5 wt % of K₂O,10 to 16 wt % of R₂O (wherein R₂O=Li₂O+Na₂O+K₂O), 0 to 3.5 wt % of MgO,1 to 7 wt % of CaO, 0 to 2 wt % of SrO, 0 to 2 wt % of BaO, 2 to 10 wt %of RO (wherein RO=MgO+CaO+SrO+BaO), 0 to 2 wt % of TiO₂, 0 to 2 wt % ofCeO₂, 0 to 2 wt % of Fe₂O₃, 0 to 1 wt % of MnO and 0.01 to 3 wt % ofTiO₂+CeO₂+Fe₂O₃+MnO.

With respect to the glass substrate, it is preferred that a hole isbeforehand opened at a central portion of the glass substrate by use ofa grinding stone to provide a disk-shaped glass substrate having a roundhole at the center thereof.

It is also preferred the glass substrate is beforehand chamfered atouter and inner peripheral end faces thereof. Preferably, the glasssubstrate is previously ground to a given surface roughness at the outerand inner peripheral end faces and the main surface thereof.

(Pre-Polishing Step)

In the pre-polishing step, the glass substrate is relatively roughlypolished on the surfaces thereof to quickly remove defects and strainsfrom the surfaces of the glass substrate. In this sense, the mirrorpolishing step differs from a mirror polishing step in respect of apolishing pad, a polishing liquid and polishing grains used.

In the pre-polishing step, it is preferred to use a relatively hardpolishing pad. Moreover, it is also preferred that particles having anabrasive action such as, for example, zirconium oxide particles and/orcerium oxide particles are contained beforehand.

According to the studies made by us, in a pre-polishing treatment(pre-polishing step) of a glass substrate carried out prior to themirror polishing treatment of the glass substrate for magnetic disk, ithas been found that polishing grains contained in a polishing liquid ispreferably set to be within a certain range of grain size.

More particularly, in the practice of the invention, the pre-polishingtreatment of a glass substrate carried out prior to the mirror polishingtreatment is most preferably performed in such a way that a polishingpad containing zirconium oxide particles and cerium oxide particles isused and a maximum grain size of cerium oxide polishing grains containedin the polishing liquid is set at 4 μm or below. On the other hand, thelower limit of the grain size is preferably determined while taking intoaccount a polishing rate in the pre-polishing step. Water can be usedfor a polishing liquid.

When using a polishing liquid wherein a maximum grain size is regulatedat 4 μm or below, the glass substrate is suppressed from sufferingdefects in the surfaces thereof in the course of the pre-polishingtreatment. Accordingly, in the mirror polishing treatment carried outafter the pre-polishing treatment, if the glass thickness of the glassto be removed from the glass surface is made small, a desired smoothmirror surface can be obtained. This leads to a shortened polishing timerequired for the mirror polishing treatment.

(Arrangement of a Glass Polishing Apparatus)

Subsequently, an arrangement of a polishing device 10 used to carry outthe method of making a glass substrate for magnetic disk according tothe invention is illustrated with reference to FIG. 1. FIG. 1 shows asectional arrangement of a polishing device for carrying out the methodfor making a glass substrate for magnetic disk according to anembodiment of the invention.

As shown in FIG. 1, the polishing device 10 is so arranged as tosandwich a glass substrate 1 to be polished between an upper platen 3 aand a lower platen 3 b through polishing pads 2. The polishing device 10is arranged to sandwich a plurality of glass substrates 1 at the sametime.

The upper platen 3 a, lower platen 3 b and glass substrate 1 can berelatively moved in horizontal directions. Such movements are possibleby use of planetary gear mechanisms built in the upper platen 3 a andthe lower platen 3 b, respectively. It will be noted that the upperplaten 3 a and the lower platen 3 b can be moved while adding a givencompression pressure against the glass substrate 1.

The upper platen 3 a and the lower platen 3 b are preferably constitutedof a material having a corrosion resistance to acids, respectively. Forinstance, martensite stainless steels or austenite stainless steels arepreferred as a stainless steel having an excellent corrosion resistance.

It is preferred that the hardness of a polishing pad is appropriatelycontrolled depending on the polishing rate and the surface roughness.For example, when mirror polishing is effected, a relatively softpolishing pad 2 is preferably used so as to obtain a smooth mirrorsurface suited as a glass substrate for magnetic disk. On the otherhand, in order to attain a high polishing rate, a relatively hardpolishing pad 2 is preferably used.

(To Carry Out a Mirror Polishing Step)

Subsequently, a specific example of a method for carrying out a mirrortreatment process using the polishing device set out above isillustrated.

Initially, the glass substrate 1 is set in the polishing device 10. Moreparticularly, the glass substrate 1 is sandwiched between the upperplaten 3 a and the lower platen 3 b through the polishing pads 2 tobring the glass substrate 1 into contact with the polishing pads 2 atopposite surfaces thereof.

Next, a polishing liquid containing such polishing grains as set outbefore is supplied to the surfaces of the glass substrate 1 to bepolished.

Subsequently, the glass substrate 1 and the upper platen 3 a and lowerplaten 3 b are relatively moved, so that the glass substrate 1 and thepolishing pads 2 are relatively moved thereby polishing the glasssubstrate 1 on the opposite surfaces thereof.

The polishing liquid in this mirror polishing step may be re-utilized bycirculation. More particularly, a once employed polishing liquid may becollected and cleaned by filtering, followed by supplying the surfacesof the glass substrate again. In the first embodiment, since the pHvalue of the polishing liquid is kept by the action of such a bufferagent as set forth hereinbefore, the polishing liquid can be re-used.

When the surface roughness of the glass substrate arrives at a givenvalue, the mirror polishing step is completed. An intended surfaceroughness is, fro example, such that an arithmetic mean roughness (Ra)is at 0.3 nm or below with a maximum peak (Rp) being at 2 nm or below.The maximum peak (Rp) means a height obtained by measuring a surfaceprofile in a given region of the surface of the glass substrate 1 andcalculating an average plane of the surface profile to determine aheight at the highest point from the average plane used as a referenceplane.

Thereafter, the glass substrate 1 is removed from the polishing device10 and the polishing liquid and polishing grains are cleaned away fromthe surface of the glass substrate 1.

Next, the thus cleaned glass substrate 1 is subjected to chemicalreinforcement, followed by cleaning the glass substrate to complete thefabrication of the glass substrate for magnetic disk.

(Glass Substrate for Magnetic Disk)

In the practice of the invention, it is preferred that when a smoothmirror surface formed on the glass substrate for magnetic disk isobserved through an atomic force microscope, the mirror surface shouldhave an arithmetic mean roughness (Ra) of 0.3 nm or below. The mirrorsurface having a maximum peak (Rp) of 2 nm or below is preferred. Themaximum peak (Rp) means a height that is obtained by measuring a surfaceprofile in a given region of the surface and calculating an averageplane of the surface profile to determine a height at the highest pointfrom the average plane used as a reference plane. Such a surface ensuresa glide height of 4 nm or below. More particularly, using the glasssubstrate for magnetic disk according to the invention, if a floatingheight of a magnetic head is at 4 nm, no crashing failure takes place.Because no thermal asperity is caused against a magnetic resistanceeffect element mounted in a reproducing element of a magnetic head,information can be normally recorded and reproduced.

The glass substrate obtained according to the above method is smooth onthe main surfaces thereof and is excellent in end shape. Moreparticularly, the Duboff value of the glass substrate for magnetic diskobtained by the method according to this embodiment can be set within arange of ±10 nm. This Duboff value is illustrated below.

The end shape of the glass substrate can be assessed by use of theDuboff value. The Duboff value means a maximum distance (a value asviewed in section of the glass substrate 1) that is obtained byselecting arbitrary two points along a radial direction of a disk-shapedglass substrate in the periphery of the outer or inner end portion ofthe glass substrate 1 and connecting the points with a straight line todetermine a maximum distance from the line to the surface of the glasssubstrate 1. The Duboff value may be either plus or minus depending onthe end shape of the glass substrate 1. The end shape of the case wherethe Duboff value is plus is called roll-off shape and the end shape ofthe case where the value is minus is called ski-jump shape. A Duboffvalue closer to 0 indicates an end shape being good in the region.

It will be noted that the Duboff value may also be determined, in aglass substrate having a substantially flat main surface and end facesand also having chamfered faces formed between the main surface and theend faces, as a distance from a flat face in a deviation portiondeviated from the flat face other than peripheries and existing in theperipheries within the main surface.

With respect to the end shape of the glass substrate, the Duboff valueis preferably within a range of ±10 nm, more preferably within ±7 nm andmost preferably ±5 nm. This is because when a hard disk drive using aglass substrate 1 whose Duboff value exceeds ±10 nm is made, there is ahigh possibility that a magnetic head is in contact with the magneticdisk fabricated using this glass substrate 1 and is crashed. Thepossibility of the crashing becomes greater in case of a magnetic diskof a vertical magnetic recording system. In other words, where the aboveglass substrate is used as a magnetic disk for vertical magneticrecording, the Duboff value should preferably be within ±10 nm.

The region where the Duboff value is to be measured may be arbitrarilyset so far as it is a region of an outer periphery of the main surfaceof the glass substrate, i.e. a region where head floating is impeded inthe case of a HDD disk. For instance, the measurement is made within arange of 92.0 to 96.9% from a center of a glass substrate when adistance from the center to an end portion of the glass substrate istaken as 100%.

More particularly, for example, with the case of a glass substratehaving an outer diameter size of 2.5 inches (an outer diameter of 65 mmφand a radius of 32.5 mm), points that are, respectively, on the glasssurface at a position of 29.9 mm and at a position of 31.5 mm each fromthe center of the glass substrate are connected with a straight line,and a deviation between the straight line and the surface of the glasssubstrate in the region may be determined as a Duboff value.

The Duboff value may be obtained by measuring, for example, a range of 1to 2.6 mm from the outer peripheral end toward the center using theouter peripheral end as a base point.

In order to make the Duboff value low, the mirror polishing step ispreferably performed while controlling the zeta potential of polishinggrains. More particularly, the zeta potential of polishing grains ispreferably at −10 mV or below, or +10 mV or over (i.e. the absolutevalue is in the range of not smaller than 10 mV.

In more detail, where the pH value of a polishing liquid is at 2.0, thezeta potential of polishing grains is preferably at −10 mV or below.Where the pH value of a polishing liquid is at 3.0, it is preferred thatthe zeta potential of polishing grains is at −30 mV or below.

(Effects of the First Embodiment)

According to the method for making a glass substrate for magnetic diskand the method for manufacturing a magnetic disk in the firstembodiment, the following effects (a) to (g) are obtained.

(a) According to the first embodiment, a buffer agent is contained inthe polishing liquid, the pH value of the polishing liquid is keptwithin a given range, thereby not lowering a polishing rate.Accordingly, there can be provided a productive method for making aglass substrate for magnetic disk and a productive method formanufacturing a magnetic disk.

(b) According to the first embodiment, since a buffer agent is containedin the polishing liquid, the pH value of the polishing liquid is keptwithin a given range, not lowering a polishing rate. More particularly,a once employed polishing liquid can be circulated and re-utilized.Accordingly, an amount of discharge of industrial emissions can bereduced, thus enabling a glass substrate for magnetic disk to bemass-produced in an earth friendly manner.

(c) In the first embodiment, where the zeta potential of polishinggrains in a polishing liquid is set at −10 mV or below (i.e. theabsolute value is within a range of not smaller than 10 mV), theflowability of polishing grains in the vicinity of the end portions ofthe glass substrate can be improved. The improvement in the flowabilityof the polishing grains can in turn improve the end shape of the glasssubstrate after completion of the mirror polishing step (i.e. the Duboffvalue can be made small). More particularly, there can be provided amagnetic disk and a glass substrate for magnetic disk wherein a magnetichead is able to record and reproduce information even in the vicinity ofthe outer edge of the magnetic disk.

(d) According to the first embodiment, polishing can be made such thatthe resulting glass substrate for magnetic disk has a surface roughness,for example, of 0.3 nm or below as expressed by an arithmetic meanroughness (Ra) and a maximum peak (Rp) of 2 nm or below. This enablesthe provision of a magnetic disk and a glass substrate for magneticdisk, which are able to achieve an information recording density of notless than 100 gigabits or over per unit square centimeter.

(e) According to the first embodiment, polishing can be made such thatthe resulting glass substrate for magnetic disk has a surface roughness,for example, of 0.3 nm or below as expressed by an arithmetic meanroughness (Ra) and a maximum peak (Rp) of 2 nm or below. This enablesthe provision of a magnetic disk and a glass substrate for magneticdisk, which is adapted for use with a magnetic head whose floatingheight is 8 nm or below. This eventually enables an S/N ratio of asignal received with the magnetic head to be improved, therebyincreasing a recording density on the magnetic disk.

(f) According to the first embodiment, polishing can be made such thatthe resulting glass substrate for magnetic disk has a surface roughness,for example, of 0.3 nm or below as expressed by an arithmetic meanroughness (Ra) and a maximum peak (Rp) of 2 nm or below. This enablesthe provision of a magnetic disk and a glass substrate for magneticglass, wherein a glide height of 4 nm or below can be realized. Thiseventually enables an S/N ratio of a signal received with the magnetichead to be improved, thereby increasing a recording density on themagnetic disk.

(g) According to the first embodiment, there can be provided asmall-sized magnetic disk such as of a 1.8 to inch or 1.0 to inch typeand a glass substrate suited for the manufacture of the magnetic disk.

When the pre-polishing step is carried out prior to the mirror polishingstep to beforehand remove defects and strains from the surface of theglass substrate 1, the polishing time of the mirror polishing stepbefore an intended surface roughness is obtained can be shortened.Accordingly, the productivity of the magnetic disk and the glasssubstrate for magnetic disk can be improved.

(Method for Manufacturing a Magnetic Disk)

A magnetic disk can be manufactured by successively forming anunderlying layer, a magnetic layer, a protective layer and a lubricantlayer on the surface of the glass substrate for magnetic disk fabricatedaccording to the first embodiment of the invention.

A vertical magnetic recording disk can be made by successively formingan adherent layer made of a Cr alloy, a soft magnetic layer made of aCoTaZr-based alloy, an underlying layer made of Ru, a vertical magneticrecording layer made of a CoCrPt-based alloy, a protective layer made ofa hydrocarbon, and a lubricant layer made of a perfluoropolyether on thesurface of the glass substrate for magnetic disk.

The method for making a glass substrate for magnetic disk according tothe invention is one which includes a mirror polishing treatment of aglass substrate, wherein the mirror polishing treatment includescontacting a polishing pad on a surface of a glass substrate, supplyinga polishing liquid containing polishing grains on the surface of theglass substrate and relatively moving the glass substrate and thepolishing pad to subject the surface of the glass substrate to themirror polishing treatment. The method may be so arranged that when aplurality of glass substrates are subjected to the mirror polishingtreatment, the pH of the polishing liquid is kept at a constant level.

It is more preferred to arrange such that the pH of the polishing liquidis kept at 3 or below in the course of the mirror polishing treatment.

The mirror polishing treatment is preferably carried out by moving aplurality of glass substrates, sandwiched between an upper platen and alower platen each through a polishing pad, relative to the upper platenand lower platen to mirror polish the plurality of glass substrates onopposite surfaces at the same time.

Example 1

In this example, a glass substrate made of amorphous glass was used.This glass has a multi-component glass composition, with its type beingaluminosilicate glass. This has a structure including a glass skeletonmade of a network structure of SiO and aluminium as a modifying ion. Inaddition, the glass also contains alkali metal elements.

A specific chemical composition includes SiO₂: 63.5 wt %, Al₂O₃: 14.2 wt%, Na₂O: 10.4 wt %, Li₂O: 5.4 wt %, ZrO₂:6.0 wt %, Sb₂O₃: 0.4 wt %, andAs₂O₃: 0.1 wt %. This glass was shaped according to a direct pressingmethod to obtain a disk-shaped glass. Next, using this glass disk, aglass substrate for magnetic disk is made via the following steps.

(1) Shape Processing Step

A grindstone was used to open a hole at a central portion of the glasssubstrate thereby providing a disk-shaped glass substrate having a roundhole at the central portion. Thereafter, an outer peripheral end faceand an inner peripheral end face were chamfered.

(2) End Face Polishing Step

Next, the glass substrate was polished at end faces (inner and outerperipheries) thereof by brush polishing so that a surface roughness wasat about 1 μm by Rmax and about 0.3 μm by Ra while rotating the glasssubstrate.

(3) Grinding Step

The surface of the glass substrate was ground by selecting a grain sizeof #1000 to provide a main surface having a flatness of 3 μm, a surfaceroughness Rmax of about 2 μm and Ra of about 0.2 μm. It will be notedthat Rmax and Ra were, respectively, measured by use of an atomic forcemicroscope (AFM) (nanoscope, made by Digital Instruments Co., Ltd.). Theflatness was measured by use of a flatness measuring device andindicates a distance between the highest portion and the lowest portionof the substrate surface as viewed vertically (i.e. a difference inheight).

(4) First Polishing Step (Pre-Polishing Step)

Next, a first polishing step serving as a pre-polishing step was carriedout. This step was a pre-polishing step wherein the glass substrate waspolished beforehand prior to a subsequent mirror polishing step. In thisfirst polishing step, the glass surface portions having defects andstrains formed in the surfaces of the glass substrate were removed by agrinding step.

The step was performed by use of a double-sided polishing apparatuswherein 100 to 200 glass substrates could be polished on oppositesurfaces thereof at one time. The plurality of glass substrates weresimultaneously polished on opposite surfaces thereof by moving theplurality of glass substrates, sandwiched under pressure between anupper platen and a lower platen through polishing pads, relative to theupper and lower platens. Using planetary gear mechanisms, a number ofglass substrates can be polished at one time.

The polishing pad used was a hard polisher. The polishing pad used wasone that beforehand contained zirconium oxide and cerium oxide. Thepolishing liquid contained polishing grains made of cerium oxide, inwhich coarse grains having a grain size exceeding 4 μm had beenbeforehand removed. When the polishing liquid supplied to the glasssubstrate was measured, the maximum size of the polishing grains was at3.5 μm, with an average value being at 1.1 μm and a value of D50 beingat 1.1 μm. Other polishing conditions were as follows.

Polishing liquid: made of cerium oxide (average size: 1.1 μm) and water.

Load exerted on the glass substrate: 80 to 100 g/cm².

Removed thickness of a surface portion of the glass substrate: set at 20to 40 μm.

(5) Second Polishing Step (Mirror Polishing Step)

Next, the second polishing step serving as a mirror polishing step wascarried out. This polishing step is mirror polishing of the glasssubstrate on opposite main surfaces thereof at the same time.

This process was carried out using a double-sided polishing apparatuswherein 100 to 200 glass substrates could be polished on opposite mainsurfaces thereof at one time. The plurality of glass substrates werepolished on the opposite surfaces thereof at the same time by moving theplurality of glass substrates, sandwiched under pressure between anupper platen and a lower platen through polishing pads, relative to theupper and lower platens. A number of glass substrates could be polishedat one time by use of planetary gear mechanisms.

A recycle system of a polishing liquid was operated in such a way thatthe polishing liquid supplied to the surface of the glass substrateduring the mirror polishing treatment was collected via a drain and wasagain supplied to a glass substrate after cleaning by removal of foreignmatters with a mesh filter. As a result, the pH of the polishing liquiddid not vary in the course of the mirror polishing treatment and couldbe kept substantially constant. The polishing pad used was a softpolisher (with an Asker hardness lower than a hard polisher). Thepolishing liquid was one indicated below. The platens of the polishingdevice was constituted of a stainless steel material having a corrosionresistance.

Colloidal silica grains having a grain size of 40 nm were provided, towhich there were added sulfuric acid used as an inorganic acid havingthe capability of full dissociation, and tartaric acid as an organicacid serving as a chemical solution (buffering material) having thebuffering action to prepare a polishing liquid. The polishing liquid wasadjusted in pH to 2.

It is possible to set a concentration of sulfuric acid in the polishingliquid at a level sufficient to obtain a desired pH. For instance, theconcentration is preferably from 0.05 wt % to 1 wt %. In this example,the concentration was set at 0.15 wt %. The concentration of tartaricacid in the polishing liquid is preferably from 0.05 wt % to 1.5 wt %.In this example, the concentration was set at 0.8 wt %. The content ofsilica in the polishing liquid is preferably 5 to 40 wt %. In thisexample, the content was set at 10 wt %. The balance of the polishingliquid was ultrapure water. The measurement of an electric conductivityof the polishing liquid revealed a conductivity of 6 mS/cm.

The polishing rate in the mirror polishing step was at 0.25 μm/minuteand thus, it has been found that a better polishing rate than in priorart under such conditions as set out above can be realized. It will benoted that the polishing rate was determined by dividing a reduction inthickness (machining allowance) of the glass substrate 1 necessary forfinishing into a given mirror surface by a required polishing time.

(6) Cleaning Step after the Mirror Polishing Treatment

The glass substrate was subjected to alkali cleaning by immersion in aNaOH aqueous solution having a concentration of 3 to 5 wt %. It is to benoted that cleaning was carried out while applying ultrasonic waves.Moreover, the substrate was cleaned by successive immersion in cleaningvessels of a neutral detergent, pure water, pure water, isopropylalcohol (IPA), and IPA (vapor drying). The resulting glass substrate wasobserved on the surface thereof by AFM (Nanoscope, made by DigitalInstruments Co., Ltd.), revealing no deposition of the colloidal silicapolishing grains. In addition, no foreign matters such as of stainlesssteel or iron were found.

(7) Chemical Reinforcement Step

Next, the glass substrate was chemically reinforced. For the chemicalreinforcement, a chemical reinforcing salt made of a mixture ofpotassium nitrate (60%) and sodium nitrate (40%) was provided and heatedto 375° C., followed by immersing therein the cleaned glass substratepreheated to 300° C. for 3 hours. The lithium ion and sodium ion in thesurface layer of the glass substrate were replaced by a sodium ion andpotassium ion in the chemical reinforcing salt, respectively, therebychemically reinforcing the glass substrate. It will be noted that acompression stress layer formed as a surface layer of the glasssubstrate had a thickness of about 100 to 200 μm. The glass substrateafter completion of the chemical reinforcement was quenched by immersionin a water vessel at 20° C. and maintained for about 10 minutes.

(8) Cleaning Step after the Chemical Reinforcement

The thus quenched glass substrate was immersed in sulfuric acid heatedto about 40° C. and cleaned while applying ultrasonic waves.

(9) Step of Inspecting the Glass Substrate for Magnetic Disk

The glass substrate for magnetic disk fabricated in a manner as statedabove was inspected.

The surface roughness of the glass substrate was measured by AFM (atomicforce microscope), revealing that a maximum peak Rp was at 1.8 nm and anarithmetic mean roughness Ra was at 0.25 nm. The surface was in a cleanmirror surface state. No foreign matters impeding the floating of amagnetic head and causing a thermal asperity problem were found in andon the surfaces.

Using the glass substrate for magnetic disk fabricated in theabove-stated way, a magnetic disk for vertical magnetic recordingsystems was made.

(10) Step of Making a Magnetic Disk

An adherent layer made of a Cr alloy, a soft magnetic layer made of aCoTaZr-based alloy, an underlying layer made of Ru, a vertical magneticrecording layer made of a CoCrPt-based alloy, a protective layer made ofa hydrocarbon, and a lubricant layer made of a perfluoropolyether weresuccessively formed on the surface of the glass substrate, therebymaking a vertical magnetic recording device.

(11) Step of Inspecting the Magnetic Disk

The thus made magnetic disk was inspected. A head for inspection whosefloating height was 8 nm was floated and run over the magnetic disk,whereupon it was found that the head was not in contact with foreignmatters and the like and no crashing failure took place. Next, using amagnetic head whose floating height was 8 nm and which had a reproducingelement unit of a magnetic resistance effect type and a recordingelement unit of a single polarization type, a recording and reproducingtest using a vertical recording system was carried out, confirming thatinformation could be normally recorded and reproduced. During the test,no thermal asperity signals were detected in reproducing signals.Recording and reproducing operations could be performed at 100 gigabitsper unit square inch.

Thereafter, a glide height test of the magnetic disk was conducted. Thistest is to confirm that the contact of a head for inspection with amagnetic disk occurs at what floating height in case where the floatingheight of the head for inspection is gradually lowered. As aconsequence, with the magnetic disk of this example, no contact tookplace at a floating height of 4 nm over from an inner edge portion to anouter edge portion of the magnetic disk. The glide height at the outeredge portion of the magnetic disk was at 3.7 nm.

Examples 2 to 6

In Examples 2 to 4, tartaric acid was contained as a buffer agent and acomposition of a polishing liquid in the mirror polishing step wascontrolled to change the pH value of the polishing liquid within a rangeof from 1.0 to 3.0. Other conditions were same as in Example 1. Becauseof the addition of the buffer agent, no variation in the pH value of thepolishing liquid was found with a lapse of time in the mirror polishingstep, thereby enabling the liquidity of the polishing liquid to be keptsubstantially constant.

On the other hand, in Examples 5 and 6, tartaric acid was contained as abuffer agent and the composition of a polishing liquid in the mirrorpolishing step was controlled to change the pH value of the polishingliquid within ranges of less than 1.0 or exceeding 3.0. Other conditionswere same as in Example 1. Because of the addition of the buffer agent,no variation in the pH value of the polishing liquid was found with alapse of time in the mirror polishing step, thereby enabling theliquidity of the polishing liquid to be kept substantially constant.

As a result, it was found that in Examples 2 to 6, the variation ofprocessing rate with time in the mirror polishing step could be reduced.This enables the variation of the processing rate to be lessened, forexample, in the case of mass-producing a glass substrate for magneticdisk.

In Examples 2 to 6, the relation between the pH value of the polishingliquid and polishing rate is shown in FIG. 2. According to FIG. 2, ithas been found that when tartaric acid is contained as a buffer agentand the pH value of the polishing liquid is from 1.0 to 3.0, a goodpolishing rate can be realized.

Comparative Example 1

In Comparative Example 1, no tartaric acid was contained as a bufferagent in a polishing liquid in the mirror polishing step. The pH valueof the polishing liquid was adjusted to 2.0 by controlling the amount ofsulfuric acid in the polishing liquid. Other conditions were same as inExample 1. In Example 1 and Comparative Example 1, 1000 glass substratesfor magnetic disk were, respectively, made. In Example 1, no variationof pH was found during the mirror polishing treatment, with a pH beingkept substantially constant. In Comparative Example 1, the pH increasedwith time, resulting in a reduced polishing rate. With the case ofComparative Example 1, as the batch number increased, the roughness ofthe main surface of the glass substrate became greater when comparedwith that of the glass substrate for the same batch number in Example 1.

With Comparative Example 1, the processing time required for the mirrorpolishing treatment became prolonged. In other words, with ComparativeExample 1, the pH value of the polishing liquid increased with anincreasing time of the mirror polishing step, thereby lowering thepolishing rate. More particularly, it was found that when a buffer agentwas mixed in the polishing liquid, the productivity of the glasssubstrate for magnetic disk and the magnetic disk could be improved.

[References]

Subsequently, an experimental example (reference) is illustrated whereinwhile keeping a pH value within a predetermined range (within a rangesimilar to Example 1), other conditions such as a zeta potential ofpolishing grains and the like are changed. In the following references,the references are mutually compared with one another and satisfactorilyshow the effects of the invention, and should not be construed aslimiting the invention thereto.

<References 1 to 4>

In References 1 to 3, a zeta potential of polishing grains in apolishing liquid was set at −10 mV or below in the mirror polishingstep. Other conditions were same as in Example 1.

On the other hand, in Reference 4, a zeta potential of polishing grainsin a polishing liquid in the mirror polishing step was set at −10 mV to0 mV. Other conditions were same as in Example 1.

The relation between the zeta potential of polishing grains and theDuboff value in References 1 to 4 is shown in FIG. 3. According to FIG.3, it has been found that the Duboff value is small within a range ofthe zeta potential of not larger than −10 mV and the end shape of theglass substrate 1 obtained after the mirror polishing step is good.

The Duboff value was measured by use of an electrophoretic lightscattering method after the cleaning step subsequent to the mirrorpolishing treatment. More particularly, the glass substrate (for makinga φ65 mm disk) was measured within a range of 29.9 to 31.5 mm from thecenter thereof.

When a magnetic disk for a vertical magnetic recording system was madeusing the above glass substrate 1 and subjected to such a head crashingtest and glide height test as in Example 1, similar results as in theexamples were obtained in References 1 to 3. In Reference 4, however,crashing by contact with a magnetic head took place. In view of this, ithas been found that the end shape of a glass substrate for magnetic diskis important and if the end shape is not good, head crashing takes placewhen the substrate is used as a magnetic disk.

<Reference 5>

In Reference 5, a polishing pad containing neither cerium oxide norzirconium oxide was used in a pre-polishing step. Other conditions weresame as in Example 1.

As a result, a small number of defects were left in the surface of aglass substrate 1 after completion of the pre-polishing step.Accordingly, in order to obtain a mirror surface of the same quality asin Example 1 after the mirror polishing step performed on the glasssubstrate 1, a polishing time longer than in Example 1 was required.More particularly, it has been found that when the pre-polishing step iscarried out using a polishing pad containing polishing grains such as ofcerium oxide or zirconium oxide, the productivity of the glass substratefor magnetic disk and the magnetic disk can be improved.

<Reference 6>

In Reference 6, coarse particles were not removed from cerium oxidepolishing grains contained in a polishing liquid. The measurement of thepolishing liquid revealed that the polishing grains contained in thepolishing liquid had a maximum size value of 10 μm, an average value of1.6 μm and a D50 value of 1.6 μm.

As a result, a small number of defects were left in the surface of theglass substrate 1 after completion of the pre-polishing step.Accordingly, in order to obtain a mirror surface of such a quality as inExample 1 after the mirror polishing step performed on the glasssubstrate 1, a polishing time longer than in Example 1 was required.Thus, it has been found that when coarse particles are beforehandremoved from the polishing liquid in the pre-polishing step, theproductivity of the glass substrate for magnetic disk and the magneticdisk can be improved.

Second Embodiment

Another embodiment (second embodiment) of the invention is describedbelow. It will be noted that such arrangements as in the firstembodiment are not illustrated again for convenience's sake.

According to further studies made by us, it has been found that thedispersability of polishing grains in a polishing liquid influences theend shape and surface roughness of a glass substrate after polishing.More particularly, it has been discovered that where the dispersabilityof polishing grains is poor, the polishing grains mutually coagulate,thereby worsening the end shape and surface roughness of the glasssubstrate after polishing.

We have examined the problem from various standpoints of view in orderto improve the dispersability of polishing grains in the polishing step.As a result, attention has been paid to the zeta potential of polishinggrains. It has been found that not only there is a certain relationbetween the degree of dispersion or coagulation and the zeta potentialof polishing grains, but also there is a causal relation between thezeta potential and the end shape and surface roughness.

Based on the above knowledge, we arrived, according to the invention, ata method for making a glass substrate for magnetic disk and a method formanufacturing a magnetic disk wherein a good end shape can be obtainedafter mirror polishing of a glass substrate. An embodiment of theinvention (second embodiment) based on the above knowledge is nowdescribed.

The method for making a glass substrate for magnetic disk according tothis embodiment is one which has a mirror polishing step includingcontacting a polishing pad with a surface of a glass substrate,supplying a polishing liquid containing polishing grains on the surfaceof the glass substrate, relatively moving the glass substrate and thepolishing pad to polish the surface of the glass substrate, wherein themirror polishing step is carried out while controlling a degree ofcoagulation or dispersion of the polishing liquid in such a way that aDuboff value indicating an end portion of the glass substrate obtainedby the mirror polishing step is within ±10 nm.

For a method of controlling a degree of coagulation or dispersion ofpolishing grains in the polishing liquid, there is illustrated a methodof realizing the above control by controlling a zeta potential ofpolishing grains. It will be noted that a method capable of controllinga degree of coagulation or dispersion in the polishing liquid should notbe construed as being limited to this method, and there are, forexample, many methods including a method of adding a dispersing materialand the like, and thus, the invention should not be construed as limitedto the control of zeta potential.

(Zeta Potential of Polishing Grains)

Polishing grains dispersed in a polishing liquid have a zeta potential.Where the zeta potential is closer to 0 mV than to −10 mV or +10 mV, thepolishing grains are more liable to coagulation, thereby worsening thedispersability of the polishing grains in the polishing liquid. When thedispersability of polishing grains degrades, the flowability of thepolishing grains in the vicinity of end portions of the glass substrate1 lowers in the polishing step, with the possibility that the end shapeof the glass substrate 1 after the polishing step degrades. The zetapotential may be changed to either plus or minus depending on thecomposition of target particles. The zeta potential of colloidal silica(colloid-shaped silica) usable as polishing grains becomes minus at a pHof 3.0 or over, is close to zero at a pH of 2.0 to 3.0, and becomes plusat a lower pH (at a pH of below 1).

Other conditions (e.g. a type of glass substrate, a polishing device,polishing conditions and the like) are same as in the first embodimentand their illustration is omitted.

The polishing liquid was so prepared as to meet such conditions as setout hereinabove and a mirror polishing step is carried out using thispolishing liquid, thereby enabling a glass substrate for magnetic diskto be made as having an end shape more excellent than prior artcounterparts.

(Effects of the Second Embodiment)

According to the method for making a glass substrate for magnetic diskand the method for manufacturing a magnetic disk of the secondembodiment, the following effects (a) to (c) and the effects (a) to (g)attained by the first embodiment are shown.

(a) According to the second embodiment, the zeta potential of polishinggrains in the polishing liquid is set at −10 mV or below or +10 mV orover (i.e. an absolute value is within a range of not smaller than 10mV), for which the polishing grains can flow, without coagulation, inthe vicinity of end portions of the glass substrate 1 in the mirrorpolishing step. Hence, coagulated polishing grains are prevented fromstaying the vicinity of the end portions, thereby leading to an improvedend shape of the glass substrate 1 after completion of the mirrorpolishing step (i.e. the Duboff value is made small). More particularly,there can be provided a magnetic disk and a glass substrate for themagnetic disk wherein a magnetic head is able to record and reproduceinformation in the vicinity of an outer edge of the magnetic disk.

(b) According to the second embodiment, since a buffer agent iscontained in the polishing liquid, the pH value of the polishing liquidis kept within a certain range, so that the polishing rate does notlower. Accordingly, there can be provided a method for making a glasssubstrate of magnetic disk and a method for manufacturing a magneticdisk, which are high in productivity.

(c) According to the second embodiment, since a buffer agent iscontained in the polishing liquid, the pH value of the polishing liquidis kept within a certain range, so that the polishing rate does notlower. In other words, a once employed polishing liquid can becirculated and re-used. Thus, the amount of discharge of industrialemissions can be suppressed, making it possible to mass-produce a glasssubstrate for magnetic disk while taking care of the global environment.

Examples

Subsequently, examples related to the above embodiments of the inventionare described along with comparative examples.

Example 1

A glass substrate for magnetic disk was made according to such aprocedure as in Example 1 of the first embodiment except that the mirrorpolishing step is replaced by the following procedure.

(Mirror Polishing Step)

A polishing device 10 capable of polishing opposite main surfaces of 100to 200 glass substrates at one time was used to carry out a mirrorpolishing step. A polishing pad used was a soft polisher.

A polishing liquid in the mirror polishing step was prepared by adding,to ultrapure water, sulfuric acid and tartaric acid and furthercolloidal silica particles having a grain size of 40 nm. Theconcentration of sulfuric acid in the polishing liquid was controlled toprovide a pH value of the polishing liquid of 1.8. The concentration oftartaric acid was set at 0.8 wt % and the content of the colloidalsilica particles was at 10 wt %. When the electric conductivity of thepolishing liquid was measured, the value was at 6 mS/cm, whereupon thezeta potential of the polishing grains was at −11.3 mV.

During the mirror polishing treatment, no variation in the pH value ofthe polishing liquid was found and the pH could be kept substantiallyconstant.

In this example, the polishing liquid supplied to the surface of theglass substrate 1 was collected by use of a drain and cleaned byremoving foreign matters through a mesh filter, and re-used by supply toa glass substrate 1.

The polishing rate in the mirror polishing step was at 0.25 μm/minute,and it was found that a favorable polishing rate can be realized undersuch conditions as set out above. It will be noted that the polishingrate is calculated by dividing a reduction in thickness (machiningallowance) of the glass substrate 1 necessary for finishing into a givenmirror surface by a required polishing time.

(Inspection Step of a Glass Substrate for Magnetic Disk)

Subsequently, the glass substrate for magnetic disk was inspected. Thesurface roughness of the glass substrate for magnetic disk was measuredby AFM (atomic force microscope), revealing that a maximum peak Rp wasat 1.8 nm and an arithmetic mean roughness Ra was at 0.25 nm. Thesurface was in a clean mirror surface state. No foreign matters impedingthe floating of a magnetic head and causing a thermal asperity problemwere found.

(Step of Making a Magnetic Disk)

Next, a vertical magnetic recording disk was made by successivelyforming an adherent layer made of a Cr alloy, a soft magnetic layer madeof a CoTaZr-based alloy, an underlying layer made of Ru, a verticalmagnetic recording layer made of a CoCrPt-based alloy, a protectivelayer made of a hydrocarbon, and a lubricant layer made of aperfluoropolyether on the surface of the glass substrate for magneticdisk.

(Step of Inspecting the Magnetic Disk)

Next, the thus made magnetic disk was inspected.

A head crashing test was carried out wherein a head for inspectionhaving a floating height of 8 nm was floated and run over the magneticdisk. As a result, the magnetic head was not in contact with a foreignmatter and the like and no crashing trouble took place.

Next, using a magnetic head wherein a reproducing element unit was madeof a magnetic resistance effect element and a recording element unit wasmade of a single polarization element and whose floating height was 8nm, a recording and reproducing test using a vertical recording systemwas carried out, confirming that information could be normally recordedand reproduced. During the test, no thermal asperity signals weredetected in reproducing signals. Recording and reproducing operationscould be performed at 100 gigabits per unit square inch.

Thereafter, a glide height test of the magnetic disk was conducted. Thistest is to confirm a floating height at which the contact of a head forinspection with a magnetic disk occurs by gradually lowing the floatingheight of the head for inspection. As a result, with the magnetic diskof this example, no contact took place at a floating height of 4 nm overfrom an inner edge portion to an outer edge portion of the magneticdisk. The glide height at the outer edge portion of the magnetic diskwas at 3.7 nm.

Examples 2 to 3 and Comparative Example 1

In Examples 2 to 3, the zeta potential of polishing grains in apolishing liquid in the mirror polishing step was adjusted to −10 mV orbelow. Other conditions were same as in Example 1. On the other hand, inComparative Example 1, the zeta potential of polishing grains in apolishing liquid in the mirror polishing step was adjusted to from −10mV to 0 mV. Other conditions were same as in Example 1.

The relation between the zeta potential of polishing grains and theDuboff value in Examples 1 to 3 and Comparative Example 1 is shown inFIG. 4. According to FIG. 4, it has been found that when the zetapotential is within a range of not larger than −10 mV, the Duboff valueis small, which is good for the end shape of the glass substrate 1 aftercompletion of the mirror polishing step.

The Duboff value was measured by use of an electrophoretic lightscattering method after the cleaning step subsequent to the mirrorpolishing treatment. More particularly, the glass substrate (for makinga φ65 mm disk) was measured within a range of 29.9 to 31.5 mm from thecenter thereof.

When a magnetic disk for a vertical magnetic recording system was madeusing the above glass substrate 1 and subjected to such a head crashingtest and glide height test as in Example 1, no crashing by contact withthe magnetic head took place in Examples 2 to 3. However, crashing bycontact with the magnetic head took place in Comparative Example 1. Thisdemonstrated that the end shape of the glass substrate for magnetic diskis important and when the end shape is poor, head crashing takes placeupon use as a magnetic disk.

[References]

Subsequently, experimental examples wherein a zeta potential ofpolishing grains was same as in Example 1 and other conditions such as apH of a polishing liquid and the like are changed is illustrated. Itwill be noted that the following references are mutually compared withone another, satisfactorily show the effects of the invention, andshould not be construed as limiting the invention thereto.

<References 1 to 6>

In references 1 to 4, the composition of a polishing liquid in themirror polishing step was adjusted so as to change a pH value of thepolishing liquid within a range of from 1.0 to 3.0. Other conditionswere same as in Example 1.

On the other hand, in References 5 and 6, the composition of a polishingliquid in the mirror polishing step was adjusted so as to change a pHvalue of the polishing liquid within a range of less than 1.0 orexceeding 3.0. Other conditions were same as in Example 1.

The relation between the pH value of the polishing liquid and thepolishing rate in References 1 to 6 is shown in FIG. 5. According toFIG. 5, a favorable polishing rate can be realized when the pH value ofthe polishing liquid from 1.0 to 3.0.

<Reference 7>

In reference 7, no tartaric acid serving as a buffer agent was containedin a polishing liquid in the mirror polishing step. The pH value of thepolishing liquid was adjusted to 2.0 by controlling an amount ofsulfuric acid contained in the polishing liquid. Other conditions weresame as in Example 1.

As a result, the pH value of the polishing liquid increased with a lapseof time in the mirror polishing step, thereby lowering the polishingrate. More particularly, it has been found that mixing of a buffer agentin a polishing liquid results in improved productivity of a glasssubstrate for magnetic disk and a magnetic disk.

<Reference 8>

In reference 8, a polishing pad containing neither cerium oxide norzirconium oxide was used in the pre-polishing step. Other conditionswere same as in Example 1.

As a result, a small number of defects were left in and on the surfaceof the glass substrate 1 after completion of the pre-polishing step.Accordingly, in order to obtain a mirror surface of the same quality asin Example 1 after the mirror polishing step performed on the glasssubstrate 1, a polishing time longer than in Example 1 was required.More particularly, it has been found that the productivity of the glasssubstrate for magnetic disk and the magnetic disk can be improved whenthe pre-polishing step is carried out using a polishing pad containingpolishing grains such as of cerium oxide, zirconium oxide or the like.

<Reference 9>

In Reference 9, coarse particles were not removed from cerium oxidepolishing grains contained in a polishing liquid. The measurement of thepolishing liquid revealed that the polishing grains contained in thepolishing liquid had a maximum size value of 10 μm, an average value of1.6 μm and a D50 value of 1.6 μm.

As a result, a small number of defects were left in or on the surfacesof the glass substrate 1 after completion of the pre-polishing step.Accordingly, in order to obtain a mirror surface of the same quality asin Example 1 by performing the mirror polishing step on the glasssubstrate 1, a longer polishing time than in Example 1 was required.More particularly, it has been found that to remove coarse particlesfrom a polishing liquid beforehand prior to the performance of thepre-polishing step leads to improved productivity of a glass substratefor magnetic disk and a magnetic disk.

1. A method of making a glass substrate for a magnetic disk, comprising a mirror polishing step which includes contacting a polishing pad with a surface of a multi-component glass substrate, supplying a polishing liquid containing polishing grains to the surface of said glass substrate, and relatively moving said glass substrate and said polishing pad to polish the surface of said glass substrate, said magnetic disk allowing a magnetic head to fly at a floating height of 8 nm or less, wherein: colloidal silica grains having a grain size of not larger than 80 nm are used as the polishing grains; said polishing step being carried out under the conditions that the polishing liquid contains an inorganic acid and a buffer agent for preventing change in pH of the polishing liquid, that the pH value of the polishing liquid is kept at 3.0 or less, and that a zeta potential of said polishing grains contained in the polishing liquid is not higher than −24.7 mV, in order to control a degree of coagulation or dispersion in said polishing liquid so that a Duboff value which shapes an end contour of the resulting glass substrate obtained by the mirror polishing step falls within ±10 nm.
 2. The method of making a glass substrate for a magnetic disk as defined in claim 1, wherein the multi-component glass is an aluminosilicate glass.
 3. The method of making a glass substrate for a magnetic disk as defined in claim 1, wherein said buffer agent is composed of an organic acid.
 4. The method of making a glass substrate for a magnetic disk as claimed in claim 1, wherein the mirror polishing step includes polishing the glass substrate so that the surface roughness of the glass substrate is 0.3 nm or less in arithmetic mean roughness (Ra) and is 2 nm or less in maximum peak (Rp).
 5. The method of making a glass substrate for a magnetic disk as claimed in claim 1, further comprising a step of pre-polishing the glass substrate using cerium oxide grains having a grain size of less than 4 μm.
 6. The method of making a glass substrate for a magnetic disk as claimed in claim 1, wherein the mirror polishing step is carried out while the zeta potential is kept at −30 mV or less.
 7. A method of manufacturing a magnetic disk, wherein a magnetic layer is formed on the glass substrate manufactured by the use of a method of making a glass substrate for a magnetic disk as defined in claim
 1. 